The present invention relates to the recovery of fluoride ions from waste fluorosilicic acid solution and, more particularly to a process for converting waste fluorosilicic acid into hydrofluoric acid.
Divalent fluorosilicates (SiF.sub.6.sup.2-) are a by-product associated with wet process phosphoric acid plants. More specifically, during the production of phosphoric acid, relatively large amounts of fluoride vapors are evolved from the acidulation circuit, the evaporation operation and from other processing steps. To avoid discharge of this fluoride into the atmosphere, scrubber systems are utilized throughout the plant. Furthermore, large amounts of fluoride vapors are evolved in the phosphoric acid evaporators and are collected in barometric condensers. The water used in the scrubbers and barometric condensers, as well as water used to slurry the gypsum for disposal, is discharged into gypsum and cooling pond systems.
Where phosphoric acid is further processed through ammonium phosphate or superphosphate plants to produce a variety of fertilizer products, still more fluoride is evolved. Once again, to prevent escape of the fluoride gases into the atmosphere, scrubbers are employed and the fluorides returned to cooling ponds.
To recover fluoride from the pond water system described above, the pond water can be diverted to a fluoride recovery plant and, upon removal of the fluoride from the water, returned to the pond. Thus, in U.S. Pat. No. 4,734,200, there is described a process for the removal of fluoride contaminants from an acidic process wastewater. More specifically, it was discovered, unexpectedly, that fluorides such as SiF.sub.6.sup.2- could be removed from an acidic process wastewater by contacting the wastewater at pH levels between about 1.5 and 2.0 with a strong base ion exchange resin. Such SiF.sub.6.sup.2- loaded resin was then regenerated by contacting it with a compound capable of stripping the fluoride from the resin and forming water-soluble commercially useful end products or intermediates capable of being converted to commercially useful end products. Included among such compounds were sulfuric acid, ammonium sulfate, and ammonium bisulfate, the particular compound used not being critical so long as the fluoride salt thereby formed was water-soluble thereby enabling it to be washed out of the resin. The pH of the solution containing the ammonium silicofluoride strip solution was then raised, e.g., by adding ammonia, to increase the pH of the solution to between about 8.5 and 9.0 thereby precipitating an activated silica compound. After removing the silica, the filtrate solution including ammonium fluoride was then used as an intermediate for various commercially useful fluoride compounds such as calcium fluoride, hydrofluoric acid, metallic fluorides and the like.
The preparation of hydrofluoric acid from the wastewater is particularly advantageous because, while simultaneously removing a contaminant from a wastewater, a commercially valuable end product is produced. Additionally, the ability to produce hydrofluoric acid from an acidic process wastewater is an attractive alternative to current commercial methods for producing hydrofluoric acid. More specifically, hydrofluoric acid is generally produced by reacting sulfuric acid and high grade calcium fluoride (fluorospar). The process typically involves the intimate mixing of the fluorospar and sulfuric acid to initiate the chemical reaction wherein the calcium fluoride and sulfuric acid mixture is converted to hydrogen fluoride and calcium sulfate. The resulting mixture is subsequently treated in a rotary kiln and hydrogen fluoride evolved from the reaction mass. The hydrogen fluoride gas is then treated and subsequently condensed to form anhydrous hydrogen fluoride.
The above process, although being the primary means through which hydrofluoric acid is produced, is somewhat difficult to carry out since it involves the handling of the hydrogen fluoride-laden gas throughout most of the process. Additionally, the cost and availability of fluorospar are subject to wide and unpredictable fluctuations and thus, elimination of fluorospar as a starting material could be highly advantageous since it is generally the most significant factor affecting commercial production of hydrofluoric acid.